In recent years, an increasing number of wells are being drilled to depths that result in bottomhole static temperatures (BHST) in excess of 400° F. In order to increase the productivity of these wells to enable economic production of hydrocarbons, well stimulation operations may be necessary. A commonly employed stimulation technique is hydraulic fracturing, wherein fracturing fluids are pumped into the formation at high pressure to crack the formation and create fractures, and subsequently deposit large quantities of sand or synthetic ceramic material (termed ‘proppant’) into these fractures to keep them open. The hydraulically created fractures serve as high conductivity pathways for oil and gas to flow to the well bore at high rates. Fracturing fluids often possess high viscosities to enable cracking the formation at relatively low pumping rates, and to enable efficient transport of proppant from the surface to the fracture. The most commonly employed high viscosity fluids are obtained by mixing ‘viscosifiers’ with water. These viscosifiers are typically natural polymers such as guar (and its derivatives), cellulose derivatives, xanthan, diutan, etc. Polymer-free viscosifiers such as viscoelastic surfactants (VES) are also commonly utilized. The performance of all these viscosifiers is limited in most cases to wells having bottomhole temperatures of 300° F., and in some cases, temperatures up to 350° F. Synthetic polymers and copolymers of acrylamide, methacrylamide or 2-Acrylamido-2-methylpropane sulfonic acid, AMPS, such as polyacrylamide, partially hydrolyzed polyacrylamide and its copolymers and derivatives have been used to provide high viscosity aqueous fluids that are stable above 400° F. However, these fluids are known to be very sensitive to even small amounts (a few percent by weight) of dissolved salts.
In a number of cases, these high temperature reservoirs are composed of sandstone that may not be consolidated. In unconsolidated formations, there is a tangible risk of sand particles coming loose during production and flowing into the wellbore along with the produced fluid. These sand particles can cause erosion to production as well as surface equipment such as tubing, chokes, valves, etc. Produced sand can also bridge off the tubing, shutting down production. These events result in very high costs associated with clean out and disposal of sand, and workover to bring the well back on production. As a result, sand control techniques are employed when producing from unconsolidated or weakly consolidated formations. The simplest sand control completion is a “stand alone screen”, wherein a screen is installed on the outside of the production tubing to filter out the sand before it can be produced. Another sand control technique is placing a “gravel pack”, wherein strategically sized gravel particles are placed in the near wellbore region, outside the screen. In this scenario, the completion has two filters against sand production—the gravel pack and the screen. The gravel pack can be placed in the producing zone by pumping gravel with water or brine at high rates, and using the velocity of the fluid to transport gravel. Alternately, viscous fluids can be used to suspend and transport gravel at relatively low rates. The commonly used viscosifiers in gravel packing operations include hydroxyethylcellulose (HEC), xanthan gum, and viscoelastic surfactants (VES). As in the case of hydraulic fracturing fluids, the upper temperature limit of these biopolymer and VES fluids is usually around 300° F. or so, and in some cases, around 350° F. or so.
Additionally, another sand control technique is placing a “frac and pack”, wherein strategically sized gravel particles are placed in the near wellbore region, outside the screen, after a short hydraulic fracture has been created in the formation. In this scenario, the completion has three filters against sand production—the propped fracture, the gravel pack and the screen. The frac and pack is typically placed in the producing zone by pumping gravel with viscous fluids used to suspend and transport gravel at relatively low rates. The commonly used viscosifiers in gravel packing operations include Guar gum and its derivatives, and viscoelastic surfactants (VES). As in the case of hydraulic fracturing fluids, the upper temperature limit of these biopolymer and VES fluids is usually around 300° F. or so, and in some cases, around 350° F. or so.